So my "farm truck" is idleing very rough, to the point that sometimes that it stalls out. At high idle it runs fine but when its at a idle its very rough. Its been running fine until probably a month ago or so and its just getting worse. Any idea what it could be? Im thinking it might be that something is dirty but i just dont know where to start with figuring out what the problem is. Im up for any ideas. Thanks.

 

Vacuum leak.

 

where should I start to look for the leak?

 

A vacuum leak could be anywhere along your system. But I would check your IAC valve, ( Idle Air Control). There are a lot of threads here discussing the IAC and some actually are able to clean it although it is always better to replace. Have a look at it first though so you aren't throwing parts at a problem. In the mean time, you can adjust the idle screw on the throttle body which controls how far the throttle goes. I had to adjust mine and it idles between 600-700 RPM in park and 400-500 RPM in gear. Any less, and it tends to stall.

 

Vacuum leak will set code, is your check engine light on? If so draw the code(s) and go from there.
I.A.C. is a good guess if no code.

 

Ya the engine light is on, but its kinda been on for a while and running fine. Usually i would have it looked at right away but its a farm truck so i didnt really want to get it into town to get it looked at when it was running fine. I think i might have to cause we cant keep driving it like this.

 

The 'probable' codes are 171/174.
This is indicating excess unmeteed air after the mass air meter.
Do a good look at all the rubber parts attached to the intake in any location for hardening, splits and deterioration.
Move them by hand not just look so you catch any issue.
The engine vacuum opens can open a fault up and draws air in.
Idle is the most sesnitive point so you notice is as poor idle quality.
The observation that the motor runs at higher rpm seemingly ok just fools you.
I'm sure you will find the causes.
Good luck.

 

While you're in town pick up a code reader if not a scan tool, even a cheapie that reads codes only is worth the small investment..

 

YOU REALLY NEED TO PULL THE CODES. If not, you end up throwing parts at a problem. You don't want to have the bed loaded with hay and a full hay trailer hooked up. And have it die in the field! An EGR valve can cause a rough idle. When does it start? After warmed up? From a cold start? Look at the plugs and wires? All the ground wire connections cleaned up and protected with dielectric grease? Nothing unplugged? Had a new fuel / air filter lately?

 

I have a 97 light duty F250 with a 5.4L and had the same issue. After posting the problem, some one advised me to check the PVC Valve hose located on the back side of the motor. sure enough the hose had a hole in it. Replaced it for about $5. cured my problems.... so take a look at it.
Chris...